The invention resides in a supply arrangement for supplying an exhaust gas system with a solution including a reducing agent in particular uric acid. The invention is also concerned with an internal combustion engine including such an exhaust gas cleaning system and also with a power generator unit and further with a method for supplying a solution containing a reducing agent to an exhaust gas system of an internal combustion engine and to a control arrangement.
For complying with future emission laws for diesel engines, treatment of the exhaust gases is unavoidable. In one particular exhaust gas treatment a liquid reduction agent such as a solution containing uric acid is admixed to the exhaust gas. The liquid reduction agent selectively reduces nitrogen oxides (NO, NO2). This may occur by catalytic or non-catalytic reaction. For performing the respective reaction ammonium (NH3) is required (N2) so that as product of the reaction water (H2O) and nitrogen (N2) is formed. In case of a selective catalytic reaction, catalysts on the basis of zeolytes or essentially on the basis of titanium dioxide, vanadium pentoxide and tungsten oxide may be used.
The ammonium necessary for the selective reaction is generally not used in pure form but for example in the form of a 32.5 percent aqueous urea solution whose composition is defined by DIN 700-70. Basically, the terminology of a liquid reaction agent in connection with the present application is to be understood broadly and comprises in particular all liquid compositions which can provide ammonia for a reaction for reducing nitrogen oxides. In the preferred case of an aqueous urea-containing solution ammonia and CO2 is formed by a hydrolysis reaction. The ammonia generated in this manner can react in the exhaust gas with the nitrogen oxide at a certain temperature in a catalytic or non-catalytic reaction.
Mainly dependent on the method of a catalytic or non-catalytic reaction, the reduction agent is admixed to the exhaust gas in a suitable quantity. The quantity of the admixed reduction agent has been found to be critical to obtain an effective reduction of the nitrogen oxides depending on the load, the selected method, and particularly the selected catalyst, etc. To this end, the reduction agent supply arrangement includes a dosing device for controlling the quantity of the solution containing the reducing agent as needed to an injector for injecting the reducing agent into the exhaust gas system. The amount of for example aqueous urea solution needed for a selective catalytic reaction is, depending on the raw emission of an engine, about 2% to 8% of the Diesel fuel used. A reserve of the reduction agent is stored in a storage tank. A reduction agent quantity as needed for the injection is provided in an operating tank which is directly connected to the dosing device so as to form an intermediate store which is rapidly available when needed.
It is however problematic that a solution containing a reduction agent freezes at low temperatures. A solution containing urea freezes for example at −11° C. and consequently needs to be protected when the Diesel engines are used at low temperatures. During larger shut-down periods of an internal combustion engine in a cold environment, freezing of the solution containing the reducing agent particularly in the operating tank cannot always be prevented so that, before engine startup, the components containing the reduction agent need to be heated. As described for example in WO 2008/080691, the operating tank is preferably provided with a heating device so that the reducing agent provided for an immediate dosing is protected from freezing and can be supplied via the dosing device to the injector immediately after the start of the internal combustion engine.
Problematic however are the provisions needed in an engine compartment for the heating, such as the installation space, since the heating device is subject to certain installation conditions with regard to installation location and installation level with respect to a cooling circuit of the internal combustion engine. In addition, the control of the heating device has generally been found to be comparatively expensive.
An improved concept which would effectively prevent the freezing of the reduction agent in the operating tank would consequently be desirable and it is therefore the object of the present invention to provide an arrangement and a method by which an exhaust gas system of an internal combustion engine can be supplied with a reduction agent containing solution wherein a freezing of the solution in the operating tank is essentially prevented. The arrangement and method to achieve this result should also be simple and inexpensive.